Of the various types of known RF amplifiers, Class A, B, and AB amplifiers are inefficient compared to Class C amplifiers. Because of their efficiency, Class C amplifiers are often used in radio transmitters. By definition, a Class C amplifier is biased so that its quiescent state is "off". Upon receiving an input RF signal, the Class C amplifier generates a somewhat distorted, amplified RF output signal, and a tank circuit tuned to the desired output frequency effectively re-shapes the amplified RF signal.
Class C amplifiers can have extremely fast self-activation or "turn-on" times. Once the amplifier receives an input RF signal, the rise time of the output RF signal envelope can be extremely fast. The rise time is defined for purposes of the present invention as the time period it takes for a transmitted RF signal to increase from essentially zero power to full transmitter power. A graph of radio transmitter power in decibels (dB) versus time (msec) is shown in FIG. 1A. For purposes of illustration, only the envelope of the output RF signal is shown. The transmitter is turned "on" at approximately the 5 millisecond mark on the illustrated time scale, and the transmitted signal increases from an average quiescent noise level of -80 dB to full RF carrier power, characterized as zero dB, in a few microseconds. In fact, the rise time of the Class C amplifier signal response is so fast that the RF envelope is nearly a step function.
The faster the rise time, e.g. the steeper the slope of the RF envelope, the wider the transient frequency spectrum effected by that signal. Conversely, the slower the rise time or the smaller the envelope slope, the narrower the band of transient frequency spectrum effected. The presence of RF energy outside of an assigned frequency band may spill over into adjacent channels and cause interference with other signal transmissions.
In radio frequency communication applications, different transmitters may be assigned to specific channels corresponding to particular frequency bands in the RF spectrum. Excessively abrupt activation of a transmitter assigned to one channel (having a preset bandwidth) may cause transient splatter in the RF spectrum and be detected as transient noise in adjacent channels. In fact, such transmitter turn-on noise may interfere with several (e.g. five or more) adjacent and nearby channels.
An example RF envelope of a radio transmitter using channel 1 is shown in FIG. 1A. The effect on an adjacent channel 2 of turning on that transmitter is illustrated in FIG. 1B. At the time the transmitter broadcasting on channel 1 in FIG. 1A is activated (at approximately the five millisecond mark on the illustrated time line), a sharp noise spike occurs in the adjacent channel 2. This noise spike is only 5 dB below full power and may even trip the squelch circuits of sensitive receivers tuned to channel 2. In any event, such a noise spike may cause significant errors in information then being transmitted over channel 2, especially if the information is digital data. During voice transmissions over channel 2, listeners may experience such cross-channel noise spikes as loud clicks or pops on the channel.
This kind of interference is even more of a problem in certain radio applications. In conversations carried on between two portable radio transceivers, for example, each transceiver may be activated many times over a short time interval. In other words, every time a user wishes to talk, that user activates his transmitter causing interference to one or more adjacent channels.
Prior art transmitter designers have recognized the need to control amplifier gain. However, prior art amplifiers fail in some cases to sufficiently suppress interchannel noise caused by transmitter turn-on transients. This is especially so at higher carrier frequencies with closely spaced channels. There is need for an economical and effective mechanism that can be incorporated into typical RF transmitter/amplifier circuitry to control the speed with which radio transmitters turn on so as to minimize such interference effects on adjacent channels.
The present invention provides a simple and effective mechanism for reducing such transient noise caused by radio transmitter activations. By connecting an "RF snubber" circuit in parallel with the transmitter output, RF envelope rise time may be slowed considerably. As a result, adjacent channel interference is substantially eliminated.
In brief summary, an RF envelope detector can be used in conjunction with an RC circuit to self-generate a transient control circuit during abrupt envelope increases. In turn, the transient control circuit automatically and temporarily functions as an RF attenuator to slow the actual envelope rise time. Suitable static discharge resistance may also be used to reset automatically the circuit for use at the next transmitter activation.
An exemplary RF snubber circuit takes advantage of the operational characteristics of a PIN diode at radio frequencies. Initially, the PIN diode functions as a half wave rectifier to generate a DC current which is accumulated on a small first series-connected capacitor. In other words, the PIN diode and capacitor act as an envelope detector. As the RF envelope increases, the envelope detector automatically powers an RC time delay circuit. In particular, DC current flows through an RC circuit (a series connected resistor and a second capacitor) shunt-connected across the first capacitor to result in a sustained yet transient biasing current for the PIN diode. This causes the PIN diode to temporarily act like a resistor with respect to the transmitted RF signal. The first capacitor acts as an RF bypass for the RF signal, and thus, effectively connects the PIN diode directly across the RF amplifier output. The shunt PIN diode resistance loads the amplifier output and temporarily attenuates it.
The second capacitor through which the biasing current flows is, in this exemplary embodiment, large relative to the value of the first (RF bypass/charging capacitor) that couples the diode to ground. Consequently, the second capacitor maintains a diode bias current for a significant time interval, and in the process, slows the rise time of the RF envelope across the now resistive PIN diode. In other words, the first capacitor acts to keep the PIN diode effectively connected in shunt across the RF output while the RC time constant of the biasing current through second capacitor keeps the PIN diode resistive. Consequently, the rise time or turn-on characteristic of the RF envelope is reduced or slowed sufficiently to prevent spurious transient turn-on interference in adjacent channels.
The PIN diode of the present invention does not require an externally controlled bias current in order to achieve the desired resistive characteristic at transistor turn-on time. The bias current is self-adjusting using simple, passive circuit components. Once the transmitter RF envelope has reached a steady or constant power level, the capacitors reach a steady state dc voltage and the PIN diode bias current automatically disappears --thus removing undesirable RF loading and attenuation during ongoing transmitter operation. No external voltage source or control mechanism is required to actively bias the PIN diode; no external device or signal is required to remove the biasing current.